1. Field of the Invention
This invention relates to an alternating current welding apparatus that includes unique circuitry for initiating a direct current-electrode positive arc. This eliminates the use of a continuous high frequency or complex auxiliary power supplies at low current welding.
2. Background
In one type of electric arc welding commonly referred to as Tungsten Inert Gas (TIG) welding, an arc is struck between a non-consumable electrode, for example, a tungsten electrode, and a workpiece in the presence of an inert shielding gas. It is desirable to initiate the welding arc without physical contact between the electrode and the workpiece. This is most commonly accomplished by coupling a high voltage, low current, high frequency signal to the electrode. The high voltage ionizes the shielding gas and provides a path for arc initiation. There are a number of problems with this method, including electrical interference with electronic equipment in the vicinity of the arc, arc wander, and erosion, particularly of aluminum workpiece surfaces. When welding with direct current, the high frequency signal is required only to initiate the arc, so it is often turned off a short time after the arc is initiated. It may be automatically reapplied if the arc goes out.
Since the high frequency signal is a serious source of interference, a high voltage impulse has, in some instances, been used in place of the high frequency signal. The high voltage impulse is often derived from the discharge of a capacitor which has been charged to a high voltage. This impulse signal is usually inductively coupled to the weld circuit by a special transformer.
Direct current TIG welding is normally accomplished with the electrode negative with respect to the workpiece. This results in the electron flow from the electrode (usually treated to enhance electron emission) to the workpiece. The electron stream heats the workpiece and results in a highly concentrated, deep penetration weld. The electrode is bombarded by positive ions in the shielding gas which clean the electrode surface of contamination. When welding some materials, particularly aluminum, there is a serious problem with rapid oxide formation on the workpiece surface. This oxide contaminates the weld, reducing the weld quality. The use of direct current with the electrode positive eliminates the surface contamination through the cleaning effect of the positive ions. This, however, results in excessive electrode heating and produces a wandering arc with shallow penetration.
To overcome these problems, alternating current is commonly used to weld materials (particularly aluminum) encountering this problem. This provides the penetration of electrode negative welding with electrode positive cleaning. Since the arc voltage and current pass through zero with each change of electrode polarity, the arc must be reignited each cycle. Reignition with the electrode negative is usually spontaneous, because the electrode is a good electron emitter with a small cross-section, the electrode and workpiece surfaces are hot, and ionized gas is present. Reignition with the electrode positive is much more difficult. The aluminum surface is not a good electron emitter, and with flat workpieces, the surface does not concentrate the electric field. With moderate weld currents and normal welding supply voltages, the arc will not reignite spontaneously. Consequently, it is customary to use a high frequency voltage to produce reignition. The continuous use of high frequency brings with it the problems of electromagnetic interference with control and communication equipment and results in a surface erosion which may reduce the strength of the weld.
Another method to reignite the electrode positive arc is to increase the arc voltage and provide initiating current by series connection of the electrode to an auxiliary power supply. The voltage required is in excess of 150 volts and significant current is available. This is an expensive solution and results in electrode voltages and current capabilities that may be hazardous to the weldor.